Field of the Invention
The invention relates to a method for actuating a fuel injector which contains a coil drive with a solenoid and a magnet armature, wherein the magnet armature can be moved along a longitudinal axis by a magnetic field which can be generated by the solenoid.
In such a fuel injector, a magnetic field, which moves the magnet armature of the coil drive along the longitudinal axis (displacement axis), is generated by suitable excitation of the solenoid. A needle of the fuel injector which, as a function of its position, closes an opening of the fuel injector or clears the opening for a certain time for the purpose of fuel injection, is connected to the magnet armature.
In what is referred to as an amplification phase, what is referred to as an amplification voltage is applied to the coil drive of the fuel injector in order to move the magnet armature as quickly as possible from its closed position into its open position. Then, a holding voltage, which is relatively low compared to the amplification voltage, can be applied to the solenoid of the coil drive of the fuel injector in what is referred to as a holding phase, in order to hold the magnet armature in its open position. The holding voltage is generally applied in the form of a multiplicity of holding pulses, with the result that a predefined holding current is set.
The voltage for driving the coil drive, referred to here as the amplification voltage, is generated with a direct voltage transformer (DC/DC transformer) from a supply voltage which is lower than the amplification voltage. The voltage which is made available by a battery in the on-board power system of a motor vehicle serves as the supply voltage. The direct voltage transformer contains a storage capacitor for supporting the voltage made available at the output of the direct voltage transformer if the consumer connected to the direct voltage transformer, i.e. the fuel injector, briefly draws a high current. The output of the direct voltage transformer is coupled to the fuel injector or the solenoid thereof.
In order to maintain the accuracy of the switching time of the fuel injector, precise voltage parameters are defined which have to be complied with during the activation of the fuel injector, which is to say during the injection of fuel into a combustion chamber of an internal combustion engine. A determining factor is a voltage drop, which is specified as the voltage drop during the time of extraction of energy from the storage capacitor. In order to keep the voltage drop small, storage capacitors with a high storage capacity and low equivalent series resistance (ESR) are preferably used. The ESR refers to the internal loss resistance of the storage capacitor. It is influenced by material, configuration and the conductivity of an electrolyte of the storage capacitor.
This results in the disadvantage of high costs since, on the one hand, storage capacitors with a high storage capacity have to be used and, on the other hand, only storage capacitors which have a low ESR can be used.